Typical examples of highly time dispersive channels include wireless systems with large bandwidth, power line communication (e.g. for Smart Grids), underwater channels etc.
Additionally, an ad hoc network can present a scenario where a number of users attempt to exchange messages. Traditionally, time dispersion poses a difficult challenge for communication systems. The currently favoured solution is typified by OFDM and SC-FDE systems (e.g. 4G mobile systems, WiFi). The OFDM/FDE system requires a Cyclic Prefix (CP), which is at least as long as the largest delay expected in the channel. The CP is inserted in front of each OFDM symbol and does not carry any useful information, which represents a waste of bandwidth. Other existing solutions include equalisation in single carrier receivers (e.g. 2G mobile systems) and rake receivers for CDMA (e.g. 3G mobile systems). In all of those solutions, time dispersion represents a hindrance to a larger or smaller extent.
MAC Layer coordination is another source of inefficiency in communications systems. The MAC protocol regulates how competing users (services) access a shared resource (e.g. a radio channel). In a standard solution only a single user can occupy a shared resource; otherwise a “collision” occurs. The most important MAC protocols include CSMAICA (e.g. IEEE 802.11x) or (slotted) Aloha. The DS-CDMA system somewhat relaxes this constraint by allowing a group of synchronised users to transmit at the same time and in the same frequency (in the same cell). However, synchronisation is very difficult to achieve in an ad-hoc network.